1. Field of the Invention
The invention relates to a method for improving the imaging properties of at least two optical elements. More particularly, the invention relates to such a method in which the relative position of the optical elements is mutually adjusted in order to improve the optical imaging. The invention further relates to a photolithographic fabrication method.
2. Description of Related Art
From EP 1 063 684 A1 it is known to determine the birefringence distribution of individual lenses inside a projection lens of a projection exposure system as a polarization-dependent perturbation. The lenses are then selected and arranged inside the projection lens so as to obtain a total birefringence whose magnitude is less than a predetermined limit value for each optical path through the projection lens. The total birefringence is in this case made up of the sum of all the birefringences of the individual lenses being analysed. Such a method is helpful when lenses need to be rejected on the basis of an intolerable birefringence distribution, but in practice does not always lead to specification values being achieved for the imaging properties of the optical elements.
Another optimisation method is known from the specialist article “The development of microlithographic high-performance optics”, Int. J. of Optoelec., 1989, 545. When optimising the imaging properties of optical systems having optical elements which are made of crystalline materials, this method provides satisfactory results only if the crystalline materials are specially selected and the optical elements are mounted without stress. Such measures are expensive.